Highly nonlinear optical waveguides are raising a lot of interest for their use with photonic devices. A desirable property of such waveguides is a large nonlinear waveguide parameter γ≅2πn2/(λAeff) to minimize the nonlinear waveguide length and thus improve the device compactness, as well as minimizing the optical power consumption. A logical approach to maximize γ is to make the nonlinear waveguide out of a material with a large intrinsic nonlinearity (i.e. large n2 parameter) and to ensure that the guided mode is strongly confined to minimize its effective surface area (i.e. small AEff). (“Nonlinear fiber optics” by Agrawal, fourth edition, Academic press, 2007).
Amongst the materials known to exhibit a large n2, chalcogenide glasses are of particular interest for device applications based on Kerr nonlinearity as they exhibit an n2 in the order of 100˜1000 times greater than that of silica, low two photon absorption and a fast response time of less than about 100 femtoseconds (fs) (refer to: R. E. Slusher, G. Lenz, J. Hodelin, J. Sanghera, L. B. Shaw, and I. D. Aggarwal, “Large Raman gain and nonlinear phase shifts in high-purity As2Se3 chalcogenide fibers,” J. Opt. Soc. Am. B-Opt. Phys. 21, 1146-1155 (2004)).
In parallel with this, waveguide structures with minimized AEff such as microtapers also provide a significant increase in the nonlinear waveguide parameter (refer to: P. Dumais, F. Gonthier, S. Lacroix, J. Bures, A. Villeneuve, P. G. J. Wigley, and G. I. Stegeman, “Enhanced self-phase modulation in tapered fibers,” Opt. Lett. 18(23), 1996-1998 (1993)).
By combining both the large n2 parameter and a small Aeff, chalcogenide microtapers made from an As2Se3 fiber tapered down to ˜1 μm in diameter have provided a high waveguide nonlinearity approaching γ=100 W−1m−1 (refer to: E. C. Mägi, L. B. Fu, H. C. Nguyen, M. R. E. Lamont, D. I. Yeom, and B. J. Eggleton, “Enhanced Kerr nonlinearity in sub-wavelength diameter As2Se3 chalcogenide fiber tapers” Opt. Express, 15(16), 10324-10329 (2007)). Although such chalcogenide microtapers have one of the currently highest waveguide nonlinearity, their use is seriously impaired because a glass wire with a diameter of 1 μm is extremely fragile. For instance, a weak air current suffices to break this unprotected microtaper. In addition, the chalcogenide-air interface of this waveguide enables evanescent interaction with the environment outside the chalcogenide wire. This is not always desirable since unwanted dust particles setting on the microtaper may lead to light scattering, losses, and signal degradation. In other instances, two such microtapers can be aligned and placed at close distance to enable evanescent wave interaction between waveguides and thus coupling. However the distance between microtapers is hardly controlled because the thin microtapers tend to attract and stick to each other when taken in close proximity. The amount of coupling is thus not controllable with such device.
There is thus a need for an improved highly nonlinear waveguide structure which at least addresses some of the above noted limitations associated with the prior art.